The present invention is directed to a method for testing specimens such as optical fibers or preforms which are used to manufacture optical fibers.
Optical fibers for optical message transmission must exhibit good optical properties. These properties include, for example, low attenuation, large numerical aperture and low pulse spread as well as a sufficiently high tensile strength. In order to protect the fiber surface against mechanical and chemical damage, the optical fibers are usually coated during the drawing process by applying a synthetic coating thereto. Frequently, however, even coated fibers will exhibit a low strength, for example, because oven particles or foreign particles will damage the fiber surface while the fiber is still in the oven chamber and because other impurities were already present in the preform from which the fiber was drawn. The strength of the fiber will depend on the frequency and the size of these defects which will be statistically distributed over the entire fiber length.
At present, the strength of the fiber is tested in the following manner. Long fibers, which are usually more than one kilometer long are generally divided into fiber pieces which are approximately 20 meters long and these fiber pieces are loaded in tension in a testing installation until they fracture. From the statistical evaluation of the tensile forces utilized to fracture the individual specimens, a prediction concerning the achievable strength and the strength to be expected in other fibers can be derived according to the so-called Weibull method.
Frequently, the fibers are subjected to a so-called screen test after coating. Thus, the fiber is conducted over rollers and is loaded with a minimum tensile force over its entire length which minimum tensile force would not break the fiber. The mechanical contact of the fiber with the rollers under these tensile stresses can lead to damage to the synthetic coating and to the fiber surface and thus leads to a reduction in the fiber strength.
In order to test preforms such as applied fiber rods from which optical fibers are to be drawn, the preforms are examined by rather imprecise methods. Examples of these methods are disclosed in an article by H. M. Presby et al, "Optical Fiber Preform Diagnostics", Applied Optics, Vol. 18, No. 1,Jan. 1, 1979, pp. 23-30.